Just do not believe everything they say. They would recommend to OC a CPU in the most useless situations just because they constantly got a "my CPU is to weak" paranoia, its almost a enthusiast-sickness.
No, as long as only a single GPU is used, a 3570K is NOT bottlenecking. Generally a OC is additional stress to a system and can decrease stability. So, a OC is something which should make sense and not used only for the bling. In the matter of your GPU it will make sense because in many games it will have some better feeling for the FPS. However, i would be careful and dont overdo it, youre already at very high OC levels. I tested both, the 7850 and 7870, my recommendation would be just to clock the 7850 at 1050/1380 and the 7870 at 1150/1380. Reason for not maxing out the RAM: The difference is just way to low and the FPS scaling is starting to decrease. Avoding unnecessary stress will mean not to raise it higher than that. There is some good reason why Nvidia is not anymore using a interface larger than 256 bit because the GDDR5 is already that fast that the bottleneck is usualy located at the core and not anymore at the RAM.
Besides, stability @temperature is totaly dependable on the core, some handle high temps well and other doesnt. The quality of a core can be measured in so many different spots that it is truly hard to make a accurate measurement. Generally, cores who can handle high temps are the most valuable ones because cooling is one of the biggest issues on high end GPUs, in term they can not handle up to 80 C (stable), its not always easy to cool it down. If so, it will come at the cost of a very noisy fan which is not pleasant at all. In my test my 7870 can handle up to 90 C at 1150/1380 without any sign of issues, which is considered a very valuable GPU. Of course that was just for testing purpose and usualy i run the GPU at lower than 80 C.
In term someone says, yes the GPU handle insane clocks and what else but it will need lesser than 60 C, then either have to add a noisy fan or a waterblock (which does instantly exclude any small form factor systems), so that GPU is only valuable for huge tower systems. So you see, it is in the same term very dependable on what system it is used for. User of SFF do need a GPU able to handle high temps and waterblock users will need a GPU able to have high potency at very low temps. But there is no general answer to "what a GPU is capable of" or "what temps its able to work stable with", every core is different! By realizing that matter, the "supertuners" out there who want to achieve the most out of a particular system, will be able to get out the most.
Besides, the stress isnt at the breaking point because you still have to set the texture filtering to high quality, then the performance may drop.
